A novel type of high-frequency magnetic resonance spectroscopy was used to investigate magnetic excitations of the molecular magnetic cluster V-15 in the frequency range from 1.5 cm(-1) to 10 cm(-1), at low temperatures, between 2.5 K and 30 K. In the optical transmission spectra, we observed absorption lines over a wide range of magnetic fields up to 6.5 T, which correspond to the Zeeman splitting of the low-lying spin states. Our frequency-domain technique, and advanced data analysis allow accurate determination of the principal values of the anisotropic g tensor, and other mode parameters, even for powder samples. The g values were obtained as gparallel toc = 1.981 +/- 0.003 and g(perpendicular toc) = 1.953 +/- 0.003, while the intrinsic linewidth was found to be Deltanu = 0.02 - 0.035 cm(-1) ( 210 - 370 Oe). The temperature dependence of Deltamu, the mode contribution to the magnetic permeability, is in good agreement with theory.